Non-linear optics

ABSTRACT

Optical elements having non-linear optical properties comprising a compound of the formula: ##STR1## wherein D is an electron donor 
     A is an electron acceptor; 
     and W, X, Y and Z are each independently H or any group capable of attachment to the pyrazolyl ring; in which the molecules of the compound are aligned so that the element has a net non-centrosymmetry, a method for their preparation, optical devices comprising such elements, and compounds of formula I.

This is a continuation of application No. 07/010,634, filed Feb. 4,1987, which was abandoned upon the filing hereof.

This specification describes an invention relating to a non-linearoptical (NLO) element and devices containing such elements.

According to the present invention there is provided an optical elementhaving non-linear optical properties, hereinafter referred to as the"NLO element", comprising a compound of the formula: ##STR2## wherein Dis an electron donor

A is an electron acceptor;

and W, X, Y and Z are each independently H or a substituent, in whichthe molecules of the NLO compound are aligned so that the the elementhas a net non-centrosymmetry, which alignment is herein after referredto as "ordered".

The electron donor, D, comprises an atom or group which readily gives upan electron, D', which preferably contains a sulphur, nitrogen, oxygenor phosphorus atom, preferably carrying one or more hydrogen atoms orhydrocarbon substituents. Examples of suitable donors represented by D'include --OR, --ST, --NTV, PTV in which T and V each independentlyrepresents H, alkyl, alkenyl, aryl or heteroaryl, or T and V togetherwith the N or P atoms to which they are attached represent ahetero-aliphatic or hetero-aromatic ring, such as morpholino,piperazino, piperidino or pyridino. Specific examples of suitable groupsrepresented by D' are OCH₃, SCH₃, SC₆ H₅, SC₆ H₄ N(CH₃)₂, NH₂, NHCH₃ andN(CH₃)₂.

One or more groups represented by D' may be attached directly to the 1-Natom of the pyrazoline ring or through a linking group, L, comprising asystem of conjugated double and single bonds between D' and thepyrazoline ring, such as a phenylene, naphthylene or alk-(polyene)-ylenegroup. The electron donor can thus be represented by the formula:

    --(L).sub.1 --(D').sub.m                                   II

wherein

L and D' are as hereinbefore defined;

l is 0 or 1;

and m is an integer from 1 to 3.

Examples of such groups are 4-(dimethylamino)phenyl

The electron acceptor, A, comprises one or more atoms or groups whichreadily accept electronic charge, A', preferably containing nitrogen andoxygen atoms, carbon and oxygen atoms, sulphur and oxygen atoms and/orhalogen atoms. Specific examples of suitable electron acceptors are--NO, --NO₂, --CN, --NC, halogen, especially F, Cl or Br, --CHO, --COOH,--COT, --COOT, --CH═NT, --CONTV, --N═NT and --C.tbd.CH, in which T and Vindependently represent H, alkyl, alkenyl, heteroaryl or aryl,especially phenyl and preferably phenyl substituted by one or moreelectron withdrawing groups, such as --NO, --NO₂, --CHO, --COOT, --CONTVand --CN. Especially preferred examples of A' are --NO₂, --CHO, --COOT,--COT, --CONTV and --CN. The electron acceptor, A, preferably alsocomprises a group containing conjugated double and single bonds, L,through which it is attached to the 3-C atom of the pyrazoline ring. Itis preferred that L represents a phenylene, naphthylene oralk-(polyene)-ylene, which may carry other substituents, includingfurther groups represented by A'. Thus the electron acceptor, A, canconveniently be represented by the formula:

    --(L).sub.l --(A').sub.m                                   III

wherein L, A', l and m are as hereinbefore defined. Examples of suchgroups are 4-cyanophenyl and 2,4-dinitrophenyl.

The groups represented by W, X, Y and Z are isolated from the conjugatedelectron pathway between the electron donor and acceptor represented byD and A and, therefore, do not significantly interfere with theelectro-optical properties of the molecule. They can represent anygroup(s) capable of attachment to the pyrazoline ring but, forconvenience of preparation, are preferably hydrogen.

Examples of suitable pyrazolines of Formula I are1-(4-methoxyphenyl)-3-(4-nitrophenyl)-pyrazoline,1-amino-4-(4-nitrophenyl)-pyrazoline,1-(4-aminophenyl)-3-(4-nitrophenyl)-pyrazoline,1-(2,4-dimethoxyphenyl)-3-(2,4-dinitrophenyl)-pyrazoline,1-phenyl-3-(4-nitrophenyl)-pyrazoline and1-phenyl-3-(4-cyanophenyl)-pyrazoline.

The molecules of the NLO compound are non-centrosymmetric by virtue oftheir polarisation along an axis through the the electron donor, thepyrazoline ring and the electron acceptor. A molecule of the compoundcan therefore be represented as a vector directed along this axis fromthe substituted electron donor, D, towards the electron acceptor, A. Amaterial, such as an optical element, comprising the NLO compound,either alone or in conjunction with other substances, in which themolecules of the NLO compound are "ordered" (i.e. not randomly orientedso that the sum of the individual molecular vectors is zero) will havean overall non-centrosymmetry and thus be adapted for non-linear opticalapplications.

The NLO element may comprise the NLO compound of Formula I alone or itmay be a medium comprising a physical or chemical combination of the NLOcompound of Formula I with other compounds which may or may not have NLOproperties.

The NLO element may comprise (i) a bulk sample of the NLO compound, suchas a single crystal prepared by crystallisation from solution, from themelt, by vapour phase transport or by other known methods ofcrystallisation, or (ii) an chemically inert medium containing the NLOcompound, such as liquid crystal material, in which the NLO compound maybe ordered by the application of a d. c. electric field. The ability ofthe NLO compound to form an "ordered" crystal is believed to be promotedby the presence in the molecule of a chiral atom which promotes theformation of crystals in which the molecules are "ordered" so that thebulk sample is non-centrosymmetric. It is therefore preferred that anNLO compound for use in the preparation of a single crystal NLO elementcontains one or more chiral atoms.

Alternatively, the NLO element may comprise a thin film of the NLOcompound on a transparent or reflecting substrate, for use inwaveguiding geometries well known in this field of work. The film mayitself be used as a waveguide, to maximise non-linear opticalinteractions, or may be used as a non-linear optically-active overcoatto an active or passive waveguide. The formation of an "ordered" filmcomprising a series of monomolecular layers, by a Langmuir-Blodgetttechnique, is believed to be promoted by the presence of long chains andthus a preferred class of the NLO compound has a long alkyl or alkenylchain, preferably as one of the terminal substituents W, X, Y or Z onthe bridging group, or the substituent V in one or more of the groupsrepresented by D' and/or A'.

The film may also be formed, for example, by epitaxial crystal growth orby crystallisation of the material in a narrow cavity between twosubstrates.

The NLO element may be employed in optical devices which exhibitsecond-order non-linear effects such as second harmonic generation,frequency mixing or the d. c. electro-optical effect.

Examples of non-linear optical effects using an NLO element inaccordance with the present invention, in the form of a bulk sample, forexample a single crystal, of the NLO compound, include:

(1) Second Harmonic Generation: A laser beam of given frequency,incident on one face of an NLO element comprising an "ordered"single-crystal of the NLO compound, at an angle parallel to theso-called "phase-matching" direction, causes the emission from theelement of a coherent beam of laser radiation, at twice the frequency ofthe incident beam, in a direction substantially parallel to the incidentbeam.

(2) Electro-optical Amplitude Modulation. A polarised laser beam isdirected so that it passes through a birefrequent NLO element,comprising an "ordered" crystal of the NLO compound, at an angle suchthat the plane of polarisation is rotated, by an angle Q, on passingthrough the crystal and then through a polarising medium (the`analyser`) which transmits a proportion of the beam corresponding to Q.An electric field, applied across the NLO element causes a change in thebirefringence (the "d. c. electro-optic effect") of the element and aconsequent change in the angle of rotation of the polarised output beam,to Q'. The proportion of the beam transmitted by the analyser nowcorresponds to Q'.

Where the NLO element comprises a thin film of the NLO compound on asubstrate this preferably comprises at least two monolayers of the NLOcompound, in which the molecules in both layers are "ordered", and morepreferably all the molecules are aligned in the same manner and such anoptical element comprises a second aspect of the present invention.

By "aligned in the same manner" is meant that the vectors along the axesof polarisation in the molecules are substantially parallel and in thesame sense.

It is not essential that the monolayers of the NLO compound are adjacentand it can be advantageous to separate the monolayers with interveninglayers of other materials. Where the two monolayers of the NLO compoundare adjacent it is preferred that the electron donors, D, of themolecules in one monolayer will be adjacent to the electron acceptors,A, in the adjacent monolayer ("head to tail" array).

Where the substrate is transparent at the wavelength of incidentradiation it may be in the form of an optical waveguide on the outersurface of which the NLO compound is deposited. With this form ofelement an optical signal passing along the waveguide interacts with thesuperficial coating of the NLO compound, via the evanescent wave whichextends into this coating, and gives rise to non-linear optical effects.Examples of suitable substances for a substrate in the form of awaveguide are glass, lithium niobate and silicon nitride on oxidisedsilicon.

Alternatively, a transparent substrate may be in the form of a plate ordisc on one, or both, surfaces of which a coating of the NLO compoundcan be formed. With this form of element a non-linear optical effect maybe obtained by transverse illumination of the substrate and film(s).Suitable substrates for such an optical element include glass, silicaand polymethylmethacrylate (PMMA).

Where the substrate is reflecting it conveniently has a plane reflectingsurface on which a superficial coating of the present NLO compound maybe formed so that the optical signal passes through the coatingimmediately before and after contact with the reflecting surface.Examples of suitable materials for the reflecting substrate arealuminium, silver, or aluminium or silver films deposited on a supportsubstrate such as glass, silica, quartz or PMMA. With this form ofoptical element it is possible to attain efficient non-linear processesby exciting the so called "surface plasmon" modes reported in theliterature [Stegman et al, Appl.Phys.Lett. 41 (10) 906, 1982; Sand etal, Appl.Optics 21 (22) 3993, 1982].

The optical element in the form of a thin layer of the NLO compound on asubstrate may be prepared by a Langmuir-Blodgett technique and accordingto a third aspect of the invention there is provided a method for thepreparation of an optical element having non-linear optical propertieswhich comprises passing a surface of a transparent or reflectingsubstrate into and out of a Langmuir trough containing a liquid carryinga superficial monomolecular layer of a compound of Formula I (the NLOcompound). Where the layers of the NLO compound are not adjacentintervening layers may be formed by passing the substrate into theliquid through a surface carrying a superficial layer of the NLOcompound and out of the liquid through another surface carrying asuperfical layer of a different compound, or vice versa.

The liquid, hereinafter referred to as the sub-phase, is preferably anaqueous medium and the mono-molecular layer or layers are maintained inthe normal manner by adjustment of the surface area with movable dams.

The optical element comprising a thin layer of the NLO compound on asubstrate is adapted for the production second order non-linear opticaleffects in a number of ways in various optical devices.

According to a fourth aspect of the present invention there is providedan optical device comprising a non-linear optical element in accordancewith the second aspect of the present invention.

An example of an optical device in accordance with the fourth aspect ofthe present invention, in which the optical element comprises asubstrate in the form of a transparent waveguide having an intimatecoating formed by multiple layers of the present NLO compound, consistsof an oxidised silicon plate having a first superficial (lower) layer ofsilicon nitride to form a superficial plane waveguide and a secondsuperficial (upper) layer comprising discrete monolayers of the NLOcompound. In operation, a first optical signal is passed through thewaveguide, (in the plane of the waveguide) and interacts with thecoating, by way of the evanescent wave which extends into the coating.This interaction generates a second optical signal, at the secondharmonic frequency with respect to the first optical signal, which canbe detected in the combined optical signal leaving the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

Another device in accordance with the present invention is described inrelation to FIGS. 1 and 2 of the accompanying drawings, in which

FIG. 1 is a plan view and

FIG. 2 is a cross-section on the line X-Y in FIG. 1. In the device theoptical element comprises a glass substrate, 4, in the upper surfaceregion, 5, of which are two transparent stripe waveguides, 6 and 8,formed in the desired pattern by the well-known ion exchange or ionbombardment techniques. The stripe waveguides are positioned to runclosely parallel over the central part of their length during which theyare separated by a distance of a few micrometers (typically 2-5 μm). Thewhole surface of the substrate, 4, is coated with a film, 9, of discretemonolayers of the NLO compound. A pair of electrodes, 10, 12, connectedto a power source, not shown, is arranged with one electrode, 10, aboveand the other, 12, below one of the stripe waveguide, 6. In operation anoptical signal is passed through the first waveguide, 6, from A to B anda voltage is applied across the electrodes. This alters the refractiveindex of the coating, due to the d. c. electro-optic (Pockels) effect,and thus the propagation constant of the first waveguide, 6. By suitableadjustment of the applied voltage the propagation constant of the firstwaveguide, 6, can be arranged so that the optical signal passing throughthis waveguide, 6, is coupled into the second waveguide, 8, and producesa second optical signal emerging from the device at C.

The optical element of the second aspect of the present invention may beused in other known forms of optical device incorporating an opticalelement by replacing the conventional NLO compound used therein, e.g.lithium niobate, with the NLO compound of Formula I.

The compounds of formula I are believed to be novel and as such form afifth aspect of the present invention.

Groups of compounds of interest include one or more of the followingfeatures:

The groups T and V within D' within D together contain no more than 18carbon atoms, and each contains no more than 9 carbon atoms.

The groups W and X, and in particular Y and Z, may if desired compriseoptionally substituted phenyl groups, for example of the type describedhereinbefore for A.

The group L, within D or A, when a system of conjugated bonds may inaddition to comprising a benzene or naphthalene nucleus or a conjugatedalkapolyene chain may also comprise an alkene moiety or a conjugatedcombination of any such groups.

In addition to suitable pyrazolines of formula I listed hereinbefore,suitable pyrazolines also include:

1-(4-dimethylaminophenyl)-3-(4-nitrophenyl)pyrazoline (2),

1-(4-dimethylamino-1-naphthyl)-3-(4-nitrophenyl)pyrazoline,

1-(6-dimethylamino-1-hexa-1,3,5-trienyl)-3-(4-nitrophenyl)-pyrazoline,

1-(4-morpholinophenyl)-3-(4-nitrophenyl)pyrazoline,

1-(4-dimethylaminophenyl)-3-(4-cyanophenyl)pyrazoline,

1-(4-dimethylaminophenyl)-3-(4-fluorophenyl)pyrazoline,

1-(4-dimethylaminophenyl)-3-(4-formylphenyl)pyrazoline,

1-(4-dimethylaminophenyl)-3-(4-carboxyphenyl)pyrazoline,

1-(4-methoxyphenyl)-3-(4-cyanophenyl)pyrazoline (3)

1-(4-methylthiophenyl)-3-(4-cyanophenyl)pyrazoline,

1-(4-phenylthiophenyl)-3-(4-cyanophenyl)pyrazoline

1-(4-4'-dimethylaminophenylthiophenyl)-3-(4-cyanophenyl)pyrazoline,

1-(4-methoxyphenyl)-3-(4-isocyanophenyl) pyrazoline,

1-(4-methoxyphenyl)-3-(4-acetylphenyl) pyrazoline,

1-(4-methoxyphenyl)-3-(4-methoxycarbonylphenyl)pyrazoline, and

1-(4-methoxyphenyl)-3-(4-phenyliminomethylphenyl)pyrazoline,

1-(4-methoxyphenyl)-3-(4-nitrophenylvinyl)-5-(4-nitrophenyl)pyrazoline(4)

1-methoxy-3-(2-nitrovinyl)pyrazoline (5),

1-(4-2'-methylbutoxyphenyl)-3-(4-nitrophenyl)pyrazoline (6),

1-(4-methoxyphenyl)-3-(2-4'-nitrophenylvinyl)pyrazoline (7), and

1-(4-dimethylaminophenyl)-3-(2-4'-nitrophenylvinyl)pyrazoline (8)

The compounds of formula I may be synthesised analogously to or arereadily and routinely derivable from known pyrazolines. For example itis often convenient to react two compounds of general form A-G and D-Jwhere D and A are as hereinbefore defined and G and J react to form thepyrazoline nucleus.

Syntheses of this type are illustrated in the Examples hereinafter, asare other aspects of the invention.

In the following Examples parts and percentages are by weight unlessotherwise indicated.

EXAMPLE 1

(a) 1-(4-Methoxyphenyl)-3-(4-nitrophenyl)pyrazoline (1)

A mixture of N,N-dimethyl-N-4-nitrobenzoylethylammonium chloride (18.1g; 0.07 m), 4-methoxyphenylhydrazine hydrochloride (12.2 g; 0.07 m) andsodium carbonate (21.2 g; 0.2 m) in ethanol (200 ml.) and water (10 ml.)was boiled for a few minutes to give a deep orange solution from which adark orange solid precipitated. The mixture was cooled to roomtemperature and the product was filtered, washed well with water and wasdried at 50° C. (14.38 g; 69% theory).

A sample was purified by recrystallisation from toluene to give greenlustrous plates.

Mpt: 138°-9° C. λCHCl₃, 471 nm (max.)

(b) 1-(4-Dimethylaminophenyl)-3-(4-nitrophenyl)pyrazoline (2)

A mixture of N,N-dimethyl-N-4-nitrobenzoylethylammonium chloride (2.58g; 0.01 m), N,N-dimethyl-p-phenylenediamine dihydrochloride(2.24 g; 0.01m) and sodium acetate trihydrate (4.08 g; 0.03 m) in ethanol (50 ml.)was heated under reflux for 30 mins.

After cooling to room temperature the mixture was acidified withhydrochloric acid and was reheated at reflux for 5 mins.

Water (100 ml.) was added, the mixture was neutralised with sodiumbicarbonate, and the solid product was filtered, water washed, and driedat room temperature (2.0g; 64.5% theory).

The product was purified by chromatography (silica/chloroform) andrecrystallisation from butanol to give a dark crystalline product.

Mpt: 215° C.

λCHCl₃, 506 nm (max.).

(c) 1-(4-Methoxyphenyl)-3-(4-cyanophenyl)-pyrazoline (3)

A mixture of dimethyl-4-cyanobenzoylethylammonium chloride (4.77 g; 0.02m), 4-methoxyphenyhydrazine hydrochloride(3.49 g; 0.02 m) and sodiumcarbonate (3.2 g; 0.03 m) in ethanol(100 ml.) and water (10 ml.) wasboiled for 5 mins. to give a yellow solution. After cooling to roomtemperature and diluting with water (100 ml.) a yellow solid productprecipitated. This was filtered and the damp paste was recrystallisedfrom butanol(2.2 g; 39.7% theory). The product was recrystallised frommethanol(1.4 g; 25.3% theory).

Mpt: 125°-6° C.

λ CHCl₃, (max).

(d)1-(4-Methoxyphenyl)-3-(4-nitrophenylvinyl)-5-(4-nitrophenyl)pyrazoline(4)

A solution of bis (4-nitrobenzal)acetone (1.62 g; 0.005 m) and4-methoxy-phenyl-hydrazine hydrochloride (0.87 g; 0.005 m) indimethylformamide (25 ml.) was heated at 100° C. for 4 hours to give adark red solution, which was drowned into water (100 ml.) The crudesolid product was filtered, water washed and was recrystallised frombutanol to give a dark red solid (1.0 g; 45% theory)

Mpt: 173° C.

Hexane 445 nm max.

The calculated second order molecular hyperpolarisability coefficients(B calc.) for compound (5) to (8) are as follows.

    ______________________________________                                                  λ (nm)                                                                           ∞                                                                              1.9    1.32 1.06                                   Compound  Ε (eV)                                                                          O      0.66   0.94 1.17                                   ______________________________________                                        (5)                 49.6          143.8                                                                              492.5                                  (6)                 54.3   83.9   151.8                                                                              443.1                                  (7)                 94.6          325.5                                                                              1760.3                                 (8)                 70.6          218.2                                                                              887.8                                  ______________________________________                                    

EXAMPLE 2

The second order molecular hyperpolarisability coefficients (β) of thecompounds of formula I are determined routinely using the well-knownEFISH (Electric Field Induced Second Harmonic) experiment described inJ. Chem. Phys., 63, 2666 (1975) on a number of solutions of thecompounds at various concentrations to determine the macroscopicsusceptibility (see also J. Chem. Phys., 67 446 (1979) and Phys, RevLett. 8 21 (1962) and using routinely determined refractive indices anddielectric constants.

EXAMPLE 3

An approximate measure of the relative efficiencies of second harmonicgeneration in potential nonlinear optical materials can be obtainedusing the powder SHG technique first described by Kurtz and Perry (J.Appplied Physics, Vol. 39, p. 3798, 1968). A polycrystalline sample ofthe material of interest is first ground in a mortar and pestle toproduce an approximately uniform particles size and a standard sizedcell (19 mm diameter×1.5 mm deep) is filled with this powder. The powdercell is mounted in the path of a Nd:YAG laser beam. The fundamentalwavelength of the laser is 1.06 um, but alternatively an incidentwavelength of 1.9 um can be obtained by passing the 1.06 beam through aHydrogen filled raman cell prior to the sample. The longer wavelengthradiation is used for samples which strongly absorb light at 530 nm(i.e. at the second harmonic of the 1.06 beam). In this case the trueefficiency of the materials could not be determined with the 1.06radiation as the second harmonic produced is reabsorbed by the material.

The laser beam (at 1.06 or 1.9 um) is incident on the powder sample, andmay result in the generation of light at the second harmonic frequency(532 nm or 950 nm respectively). The amount of harmonic generated givesa measure of the nonlinear optical efficiency of the material. Thesecond harmonic radiation is detected either in reflection oralternatively in transmission, with the photodetector placed directlybehind the sample. The sample is then replaced with a cell containingpowder of a standard material (usually urea) and the experimentrepeated. Comparison of the signals obtained in the two cases determinesthe efficiency of the material of interest relative to urea.

Typically, an incident laser energy of about 1 mJ per pulse is used, thepulse being of about 10 nsec duration, with a repetition rate of 10 H.

What is claimed is:
 1. An optical element having non-linear opticalproperties comprising a compound selected from the group consistingof:1-amino-4-(4-nitrophenyl)-pyrazoline;1-(4-aminophenyl)-3-(4-nitrophenyl)-pyrazoline;1-phenyl)-3-(4-nitrophenyl)-pyrazoline;1-phenyl-3-(4-cyanophenyl)-pyrazoline;1-(4-dimethylaminophenyl)-3-(4-nitrophenyl)pyrazoline;1-(4-dimethylamino-1-naphthyl)-3-(4-nitrophenyl)pyrazoline;1-(6-dimethylamino-1-hexa-1,3,5-trienyl)-3-(4-nitrophenyl)pyrazoline;1-(4-morpholinophenyl)-3-(4-nitrophenyl)pyrazoline;1-(4-dimethylaminophenyl)-3-(4-cyanophenyl)pyrazoline;1-(4-dimethylaminophenyl)-3-(4-fluorophenyl)pyrazoline;1-(4-dimethylaminophenyl)-3-(4-formylphenyl)pyrazoline;1-(4-dimethylaminophenyl)-3-(4-carboxyphenyl)pyrazoline;1-(4-methoxyphenyl)-3-(4-cyanophenyl)pyrazoline;1-(4-methylthiophenyl)-3-(4-cyanophenyl)pyrazoline;1-(4-phenylthiophenyl)-3-(4-cyanophenyl)pyrazoline;1-(4-4'-dimethylaminophenylthiophenyl)-3-(4-cyanophenyl)pyrazoline;1-(4-methoxyphenyl)-3-(4-isocyanophenyl)pyrazoline;1-(4-methoxyphenyl)-3-(4-acetylphenyl)pyrazoline;1-(4-methoxyphenyl)-3-(4-methoxycarbonylphenyl)pyrazoline;1-(4-methoxyphenyl)-3-(4-phenyliminomethylphenyl)pyrazoline;1-(4-methoxyphenyl)-3-(4-nitrophenylvinyl)-5-(4-nitrophenyl)pyrazoline;1-methoxy-3-(2-nitrovinyl)pyrazoline;1-(4-2'-methylbutoxyphenyl)-3-(4-nitrophenyl)pyrazoline;1-(4-methoxyphenyl)-3-(2-4'-nitrophenylvinyl)pyrazoline; and1-(4-dimethylaminophenyl)-3-(2-4'-nitrophenylvinyl)pyrazolinethemolecules of said compound being aligned so that the element has a netnon-centrosymmetry.
 2. An optical element according to claim 1comprising a single crystal of said compound or a chemically inertliquid crystal material containing said compound aligned in said liquidcrystal material by the application of a d.c. electric field.
 3. Anoptical element according to claim 1 wherein the element comprises athin film of the compound on a substrate.
 4. An optical elementaccording to claim 3 wherein the element comprises at least twomonolayers of the compound in each of which monolayers the molecules ofthe compound are aligned as defined.
 5. An optical element according toclaim 4 wherein all the molecules of the compound are aligned such thatthe vectors along the axis of polarisation in the molecules aresubstantially parallel and in the same sense.
 6. In an optical devicecomprising a non-linear optical element, the modification wherein saidelement is one as defined in claim 1.